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    <title>Document</title>
    <!-- <script>
        var age = 10;                       //这是一个数字类型
        var str = '这是一个字符串类型';     //这是一个字符串类型
    </script> -->
    <!-- <script>
        var a = 10;
        a = 'a变量由数字类型变为字符串类型';
    </script> -->
    <!-- <script>
        var age = 测试文字;   //整数
        var age = 18.5; //小数，浮点类型
        console.log(age);
    </script> -->
    <!-- <script>
        var age = 022;
        console.log(age);   //输出18
        var age1 = 0x9;     //输出9
        var age1 = 0xa;     //输出10
        console.log(age1);
    </script> -->
    <!-- <script>
        console.log(Number.MAX_VALUE);  // 1.7976931348623157e+308
        console.log(Number.MIN_VALUE);  // 5e-324
    </script> -->
    <!-- <script>
        console.log(Infinity);  //Infinity
        console.log(Number.MAX_VALUE*2);    //Infinity
        console.log(-Infinity); //-Infinity
        console.log(-Number.MAX_VALUE*2);  //-Infinity
        console.log('字符串类型'-123);  //NaN
        console.log(12-11); //1
    </script> -->
    <!-- <script>
        console.log(isNaN('数字')); //true
        console.log(isNaN(123123)); //false
    </script> -->
    <!-- <script>
        // var a = '这是一个'字符串'文本'; //报错
        // console.log(a);
        var b = '这是一个"字符串"文本';    //这是一个"字符串"文本
        console.log(b);
        var c = "这是一个'字<br/>符串'文本";    //这是一个'字符串'文本
        console.log(c);
        var d = "这是一个'字\n'符串'文本";
        console.log(d);
    </script> -->
    <!-- <script>
        alert('文本测试，到这儿需要换行了\n,然后没有然后了，只是试一试转义字符罢了！')
    </script> -->
    <!-- <script>
        var a = "这是一个   字符串，一共有18个字";     //18
        console.log(a.length);
    </script> -->
    <!-- <script>
        //字符串+字符串
        console.log('这是一个字符串'+'后面加了一串字'); //这是一个字符串后面加了一串字
        //字符串+数字的字符串
        console.log('字符串'+'110');//字符串110
        //字符串+数值类型
        console.log('字符串'+100);//字符串110
        //数值类型+数值类型
        console.log(100+100);   //200
        //数字的字符串+数值类型
        console.log('100'+100); //?
    </script> -->
    <!-- <script>
        var age1 = 10;
        console.log('测试今年age1岁了');    //测试今年age1岁了
        var age2 = 10;
        console.log('测试今年'+age2);   //测试今年10岁了
        // 引引加加 
        var age = 20;
        console.log('测试'+20+'岁');
    </script> -->
    <!-- <script>
        var age = prompt("请输入年龄");
        var str = '您今年已经'+age+'岁了';
        alert(str);
    </script> -->
    <!-- <script>
        var flag = true;    //true为真，在加法运算中为1
        var flag1 = false;  //false为假，加法运算中为0
        console.log(flag+1);    //2
        console.log(flag1+1);   //1
    </script> -->
    <!-- <script>
        var a;
        console.log(a);     //undefined
    </script> -->
    <!-- <script>
        console.log(undefined+'字符串');    //undefined字符串
    </script> -->
    <!-- <script>
        console.log(undefined+123); //NaN
    </script> -->
    <script>
        var a = null;
        console.log(a); //null
        console.log(a+'字符串');    //null字符串
        console.log(a+12);  //12
    </script>
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